As for high frequency amplifiers used for digital mobile phones, low distortion characteristics are required to prevent interference with adjacent channels. Thus, the high frequency amplifiers improve the distortion characteristics by reducing the gain compression and the phase rotation of the transistors used for amplification.
The conventional high frequency amplifier achieves the low distortion characteristics by reducing the phase rotation and the gain compression of the amplifier circuit in its entirety by connecting a diode, FET, bipolar transistor and the like to a transistor serving as the amplifying element.
FIG. 1 is a block diagram showing a configuration of a conventional high frequency amplifier. This figure shows a two-stage high frequency amplifier disclosed in Japanese patent application laid-open No. 10-135750/1998 as an example of a conventional high frequency amplifier. In FIG. 1, the reference numeral 101 designates a pre-stage bipolar transistor, 102 designates a post-stage bipolar transistor, 103 designates an RF input terminal, 104 designates an input matching circuit of the bipolar transistor 101, 105 designates a constant current source, 106 designates a power supply voltage terminal, 107 designates an interstage matching circuit, 108 designates a constant voltage source, 109 designates an output matching circuit of the bipolar transistor 102, and 110 designates an RF output terminal.
Next, the operation will be described.
The pre-stage amplifier circuit is brought into operation with keeping the base current of the bipolar transistor 101, and the post-stage amplifier circuit is put into operation with keeping the base voltage of the bipolar transistor 102. The phase rotation of the signal amplified by the bipolar transistor 101 of the pre-stage amplifier circuit is canceled out by the phase rotation caused by the bipolar transistor 102 of the post-stage amplifier circuit. Thus, as for the amplified signal output from the high frequency amplifier, its phase rotation is suppressed so that its distortion characteristics are improved.
The two-stage configuration that combines the bipolar transistor 101 whose base current is kept constant with the bipolar transistor 102 whose base voltage is kept constant can improve the distortion characteristics without inserting a new element for distortion compensation, thereby being able to reduce the number of components.
The conventional high frequency amplifier with the foregoing configuration, however, has a problem of increasing the size of the high frequency amplifier. This is because it combines at least two stages: the amplifier circuit including the bipolar transistor whose base current is kept constant; and the amplifier circuit including the bipolar transistor whose base voltage is kept constant. In addition, to improve the distortion characteristics, it is necessary to increase the emitter size of the post-stage transistor, and to set the idle current (collector-base current when a DC voltage is applied) at a large value. This offers a problem of deteriorating the power added efficiency, particularly at a low output, and of reducing the power added efficiency even at a high output.
The present invention is implemented to solve the foregoing problems. Therefore it is an object of the present invention to provide a high frequency amplifier having good distortion characteristics and high power added efficiency at the high output, and having improved power added efficiency particularly at low output power.
Another object of the present invention is to provide a compact high frequency amplifier with improved distortion characteristics and power added efficiency.